Roasting is a process whereby a food item such as a seed or nut is dry heated to a temperature which browns or caramelizes the food item for the purpose of enhancing the flavor, where the browning process includes the Maillard reaction and/or carbohydrate conversion. For the case of coffee beans, roasting is accomplished using one of several methods of heat transfer: convection, baking, and conduction, which are commonly used, or steaming of the bean, which is less frequently employed. The typical coffee bean roasting cycle involves the elevation of the beans to a temperature from 375° F. to 480° F., and lasting from 90 seconds to 30 minutes.
Convection heating as used in a fluid bed roaster, also known as a hot air roaster, is typically deployed in the form of a heated air stream which heats the beans and “floats” them in the heated air stream to impart a uniform roast and to reduce burning, with the unfortunate attendant stripping away through evaporative loss of a large amount of the coffee oils that are vital components in the flavor of superior coffee.
The conduction roasting method relies on heat from an a hot air source which heats a rotating metal drum, which in turn heats the tumbling beans through direct contact with the drum. The naturally circulating hot air, which is not mechanically convected, also heats the tumbling beans. The conduction method rotates the drum for agitation of the beans to prevent continuous contact from scorching the coffee beans. The conduction system uses air naturally circulating throughout the drum to remove heat and smoke and also results in loss of lighter coffee oils (and their flavor), as does the convection system where forced air circulation is used. The conduction system also prevents the controlled and easy transfer of the heat to penetrate the husk (which is also known as silverskin) and causes the internal mass of the beans to quickly rise to a desired temperature. This causes moisture, gases, and oil within the beans to vaporize and expand, thereby applying pressure to the beans, resulting in the popping of the cell structure of the beans, which is also known as “cracking”. The volume of the bean expands by up to approximately 50%, which frees the silverskin from the bean. As the roasting process continues, and at progressively higher temperatures, reactions involving the amino acids and reducing sugars create brown pigment typical of the Maillard reaction. Sugars caramelize and carbohydrates react, adding to the browning effect. A very lightly roasted coffee bean loses approximately 12% of its weight from an initial green bean weight, whereas a heavily (very darkly) roasted coffee bean loses up to 28% of its weight.
Steam roasting of the beans with superheated steam is another method, although it tends to produce a sour flavor, and is accordingly used less frequently. The steam roasting process uses a high-pressure vessel and the high steam temperatures and high pressures make this system potentially dangerous for the home and commercial user. Additionally, the steam system alone cannot provide the dark and very dark roasts that are desired by most of the coffee drinking public. One example of prior art steam roasting system is described in U.S. Pat. No. 5,681,607.
Convection and conduction roasting systems cause the release of steam from the green coffee bean (which typically contains 10-12% water by weight), and the steam contains latent heat, which is released upon contact with an adjacent bean. Latent heat from steam produced by convection or conduction roasting is a contributor to making the coffee have a more desirable mellow flavor than the steam-only process, but because it is an internal release of steam from the bean, it is not a hazard presented to the user of the roasting equipment.
Other problems with conductive, convection and steam roasting include roasting the bean at too low of a temperature which causes baking with a slow release of moisture from the bean, and this slow release of pressure doesn't generate enough internal pressure to crack the bean vigorously to sufficiently increase the volume of the bean for enhanced flavor. When this occurs, the roasted bean will be of smaller size than if proper roasting occurs and the improperly cracked bean will have a green grassy flavor or a baked flavor. On the other hand, if a bean is roasted at too high of a temperature, the outer surfaces of the bean will be burned, i.e., overly caramelized and carbonized, and the inner regions of the bean will be considerably less roasted, which may contribute to unwanted flavors. In some cases, high temperature roasting will result in a burning of the silverskin.
The silverskin protects the green bean in storage by helping to prevent oxidation reactions and increased moisture loss. If the roasting profile provides a slow increase in temperature and the bean does not crack properly, parts of the silverskin may remain on the bean.
The second stage of roasting occurs once the bean cracks. Here, the additional heating of the bean results in chemical changes to the roasted bean which affects the taste of the bean to particular consumers. In many instances, continued roasting of the bean after the first crack causes a further expansion of the bean and ultimately produces a second crack.
All of the above coffee roasting processes share the inability to achieve mixed degrees of roasts in a particular batch, as the convection, conduction, and steam roasting methods previously described cannot be easily stopped and restarted to produce mixed roasts without introducing new problems, such as burning of beans which stop and come to rest on the hot surfaces when the roast is paused.
Other common problems with current coffee roasters include the issue of smoke generation and excessive aroma. The smoke and excessive aroma are addressed in existing commercial roasters through the use of stack scrubbers and after-burners, and the problem is addressed on home coffee roasters by the recommended outdoor use of the roaster. Another problem of prior art convection or conduction roasters is high energy cost per pound of beans using either gas or electricity.
It is known that microwave ovens are more efficient for cooking, because the microwave energy is delivered directly to the item to be heated. The mechanism through which a microwave ovens heats a food item is through dielectric loss tangent of the absorbing food item, which loss is microwave frequency and food item dependent. Dielectric loss tangent is a measure of the dielectric loss of the medium supporting the traveling microwave. A microwave oven can operate at any frequency for which this loss tangent and dielectric absorption is high enough to cause heating, and the frequency of operation of a microwave oven is also subject to government regulation. Operational microwave oven frequencies are 2450 Mhz and the less common legacy frequency of 915 Mhz. For food items, it is desired that the dielectric loss tangent be uniform over the extent of the item to be heated. For discrete food objects such as coffee beans, this poses a problem, as the beans are both smaller in extent than a quarter wavelength of a typical oven microwave, and the discrete nature of the beans leads to hot-spot heating, with some beans in null areas, and other beans in areas of high standing wave electric fields, which generate much greater heat energy. One solution to this problem is the use of a susceptor layer, which is a local microwave RF absorptive material which is placed near the food product to be cooked. The susceptor absorbs RF, and the localized heating is coupled through a combination of radiation, conduction, and convection onto a nearby food surface. This type of material works well for large uniform cooking areas with distinct boundary areas between the region to be browned and the region to be cooked, such as low moisture content partially cooked pizza crust which is layered with comparatively high water content pizza toppings. Susceptor materials may be constructed from thin film metals or laminates of thin film conductive materials.
One prior art system used a Pyrex® tube containing coffee and closed with a rubber stopper with the enclosed volume connected to a vacuum pump, the assembly rotating in a microwave field, and tested with various levels of applied vacuum. At pressures below 6 mm Hg, coronas of ionized plasma gases appear which furnish a conducting path for electricity and result in an electric discharge, overloading of the equipment, and shutdown with some coffee beans burned in the process. High levels of vacuum could eliminate the plasma discharges, but the required vacuum cannot be drawn because of the water vapor and organic compounds drawn from the coffee under vacuum. Another problem of this system is that once the coffee is dry and temperatures exceed 300° F. (149° C.), there is sufficient localized heating which progressively concentrates on the spots of least resistance. Once carbonaceous areas form on the coffee bean, it is a good electrical conductor and the flow of excessive current in a localized spot causes electrical discharge. This problem is known as the thermal runaway problem, which arises when the power dissipation in a small elemental volume within a work piece exceeds the rate of heat transmission to its surroundings, so that the rate of increase in enthalpy is greater than in its neighbors. The temperature increases at a faster rate than in the surroundings, until decomposition occurs. Thermal runaway invariably degenerates into arcing and carbon formation, which produces profoundly undesired flavors. In the case of coffee and other low moisture foods, like nuts, seeds, dried chicory and guarana, exothermic reactions can take place in various degrees while roasting, and the problem of thermal runaway becomes more acute.
In addition to the above problems, another acute problem for standard microwave ovens is that a quarter wavelength of the 2450 Mhz traveling wave is on the order of one inch, the same length as a small clump of beans, which can cause localized electrical interactions between standing waves generated in the oven and the food items to be roasted.
The prior art and literature show clearly that the use of microwave energy for roasting has not been successfully solved because of non-uniform heating, thermal runaway, which results in carbonization followed by local arcing and plasma, and the problem of variation in level of roasting across many individual food items, as well as non-uniform roasting of any particular food item. For these reasons, the roasting of low-moisture foods (which are defined in the present patent application as foods with a moisture content less than 20%) in a microwave oven without the production of smoke, surface arcing, thermal runaway, and control of roast uniformity have long remained unsolved problems.